Ashleen J. Benson’s research while affiliated with Simon Fraser University and other places

The recovery of marine mammals from historical over-exploitation in the 1970s represents one of the largest changes in trophic structure in the northeast Pacific Ocean over the last century, for which the impacts on key forage species such as Pacific Herring (Clupea pallasii) are poorly understood. This has prompted hypotheses that increasing marine mammal populations are the primary cause for productivity declines for some fish stocks and their lack of recovery to historical abundance levels. In this study, we evaluate such a hypothesis for Pacific Herring by quantifying historical predation rates by key predators, including cetaceans (Pacific Humpbacks, Grey Whales), pinnipeds (Steller Sea Lions, Harbour Seals), and piscivorous fish (Pacific Hake). Predation mortality is quantified via a novel approach that integrates a single-species catch-at-age model with estimates of predator consumption derived from bioener-getic models. We found that predator consumption, largely driven by Humpback Whales, explained increasing Pacific Herring natural mortality rates in recent years and could be used to forecast future mortality. Incorporating higher future natural mortality rates produced higher estimates of current stock status (1.09-1.2B 0) based on lower estimates of equilibrium unfished biomass (17.5-20.3 kt). Conversely, models that assumed mortality was more like the historical average had lower stock status (0.63B 0) and higher estimates of unfished biomass (32.4 kt). We demonstrate a practical approach for ecosystem modelling that can be used to develop operating model scenarios for management strategy evaluation, improving scientific defensibility by removing an element of analyst choice for future mortality scenarios. We discuss how simpler modifications to single-species model assumptions can be more pragmatic for providing fisheries management advice, while more complex multi-species or ecosystem models might provide more nuanced insights for exploring research questions related to multi-species ecosystems and fisheries interactions.

The recovery of marine mammals from historical over-exploitation in the 1970s represents one of the largest changes in trophic structure in the northeast Pacific Ocean over the last century, for which the impacts on key forage species such as Pacific Herring ( Clupea pallasii ) are poorly understood. This has prompted hypotheses that increasing marine mammal populations are the primary cause for productivity declines for some fish stocks and their lack of recovery to historical abundance levels. In this study, we evaluate such a hypothesis for Pacific Herring by quantifying historical predation rates by key predators including cetaceans (Pacific Humpbacks, Grey Whales), pinnipeds (Stellar Sea Lions, Harbour Seals), and piscivorous fish (Pacific Hake). Predation mortality is quantified via a novel approach that integrates a single-species catch-at-age model with estimates of predator consumption derived from bioenergetic models. We found that predator consumption, largely driven by Humpback Whales, explained increasing Pacific Herring natural mortality rates in recent years and could be used to forecast future mortality. Incorporating higher future natural mortality rates produced higher estimates of current stock status (1.09-1.2B0) based on lower estimates of equilibrium unfished biomass (17.5-20.3 kt). Conversely, models that assumed mortality was more like the historical average had lower stock status (0.63B0) and higher estimates of unfished biomass (32.4 kt). We demonstrate a practical approach for ecosystem modelling that can be used to develop operating model scenarios for management strategy evaluation, improving scientific defensibility by removing an element of analyst choice for future mortality scenarios. We discuss how simpler modifications to single-species model assumptions can be more pragmatic for providing fisheries management advice, while more complex multi-species or ecosystem models might provide more nuanced insights for exploring research questions related to multi-species ecosystems and fisheries interactions.

The Spot Prawn trap fishery off the west coast of British Columbia (BC) is managed using a fixed escapement strategy that aims to prevent recruitment overfishing while maximizing expected long-term yield by closing the fishery when the catch rate of spawners, projected to the following spring, drops below 1.7 spawners per trap (the de jure rule). We develop a management strategy evaluation framework for BC's Spot Prawn fishery that examines the expected performance of the management procedure implemented in practice (the de facto rule), which was significantly more conservative than the de jure rule, usually closing the fishery when spawner catch rates were at least twice as high as specified by the de jure rule. Simulations indicate that the de facto spawner index rule using average empirical March 31 st targets from 2000-2019 maintains most stocks near or above 0.8 !"# with or without accounting for environmental effects and/or increasing future SST on recruitment. Abundance indices were found to be strongly hyperstable, with fishing efficiency 1.5-3.0 times higher under low biomass than high biomass.

Harvest control rules applied to aggregates of individual stocks or life history types can have pronounced small scale effects. Pacific Herring are currently managed as discrete stocks (5 major, 2 minor), and historical analyses of mark-recapture tagging data are consistent with this approach. Spatial distribution within discrete stocks may be both ecologically significant and important for local fishing opportunities. Neither the large inter-area nor the small intra-area scale spatial dynamics are explicitly considered in assessment and management processes. For example, within the Strait of Georgia, there has been a northward movement of spawning aggregations and an absence of spawning herring in previously occupied southern locations. Changes in herring spawning distribution have also been observed in the adjoining Puget Sound and elsewhere in British Columbia (BC). The reasons for such shifts are uncertain at this time. While the ecological processes driving herring spatial dynamics are uncertain, the cultural significance of herring is well documented. Given their importance for coastal First Nations (FN) communities, changes in spatial distribution that result in an absence of herring spawn in the waters neighboring FN communities limits access to food, social, and ceremonial harvest and cultural connection with this resource. In this talk, we will present our on-going research that evaluates: i) the management implications of spatial structure within, and connectivity among complex herring sub-populations or “stocklets”, and (ii) the potential influence of straying between major/ minor stock areas over time on our perception of herring stock status at both large and small scales.

Harvest control rules applied to aggregates of individual stocks or life history types can have pronounced small scale effects. Pacific Herring are currently managed as discrete stocks (5 major, 2 minor), and historical analyses of mark-recapture tagging data are consistent with this approach. Spatial distribution within discrete stocks may be both ecologically significant and important for local fishing opportunities. Neither the large inter-area nor the small intra-area scale spatial dynamics are explicitly considered in assessment and management processes. For example, within the Strait of Georgia, there has been a northward movement of spawning aggregations and an absence of spawning herring in previously occupied southern locations. Changes in herring spawning distribution have also been observed in the adjoining Puget Sound and elsewhere in British Columbia (BC). The reasons for such shifts are uncertain at this time. While the ecological processes driving herring spatial dynamics are uncertain, the cultural significance of herring is well documented. Given their importance for coastal First Nations (FN) communities, changes in spatial distribution that result in an absence of herring spawn in the waters neighboring FN communities limits access to food, social, and ceremonial harvest and cultural connection with this resource. In this talk, we presented our on-going research that evaluates: i) the management implications of spatial structure within, and connectivity among complex herring sub-populations or “stocklets”, and (ii) the potential influence of straying between major/ minor stock areas over time on our perception of herring stock status at both large and small scales.

Pacific herring abundance is currently assessed using a statistical catch-age-model. The catch-age model is fitted to commercial catch, proportions-at-age and fishery-independent survey index to estimate biomass and recruitment and to generate 1-year forecasts of spawning biomass (Martell et al., 2012; DFO 2014, 2015). A revised catch-at-age model was introduced for BC herring assessment in 2006 (Haist and Schweigert 2006), and the design of the model has since gone through several iterations to fix minor errors, as well as re-design and re-structuring of various model components. One major change introduced in 2011 (Martell et al., 2012) was letting the model estimate the spawn survey scaling parameter q, rather than holding it fixed at q=1, as had been done in previous iterations of the catch-age model. Another major change introduced in 2011 was to make the cut-off dependent on the model’s most recent estimate of unfished biomass, whereas it had previously been treated as a fixed quantity estimated from an earlier assessment model. Concerns have been raised regarding the application of these two changes relative to the historical management procedure (q=1 and fixed cut-offs), as well as requests to evaluate the potential consequences of these two management procedures (historical and current) using simulation. The potential consequences of these changes were not evaluated prior to their implementation, which along with lack of rebuilding in some areas has led First Nations and other stakeholders to question the appropriateness of the advice resulting from application of the current management procedure Stock Assessment and Management Advice for BC Pacific Herring: 2015 Status and 2016 Forecast. Available from: https://www.researchgate.net/publication/288670300_Stock_Assessment_and_Management_Advice_for_BC_Pacific_Herring_2015_Status_and_2016_Forecast [accessed Aug 15 2018].